Polymer electrolyte membrane and method of fabrication

ABSTRACT

A polymer electrolyte membrane comprised of a hydrophobic hydrocarbon region, a hydrophilic region containing covalently bound acid functional groups and protic functional groups. The hydrophobic hydrocarbon region and the hydrophilic region are covalently bound to form a single polymer molecule.

FIELD OF THE INVENTION

[0001] This invention relates to a polymer electrolyte membrane, andmore specifically to a membrane for use in polymer electrolyte fuelcells and related applications.

BACKGROUND OF THE INVENTION

[0002] Fuel cell technology provides for the combining of hydrogenprotons with oxygen from air or as a pure gas. The process isaccomplished utilizing a proton exchange membrane (PEM) sandwichedbetween two electrodes, namely an anode and a cathode. Membranematerials typically used for polymer electrolyte membrane fuel cells(PEMFCs) are Nafion® perfluorinated sulfonic acid polymers.Perfluorinated sulfonic acid polymers are believed to undergo microphaseseparation, which means that the hydrophilic sulfonic acid groupsassociate into separate regions from the perfluorocarbon polymerbackbone. The backbone region is hydrophobic, and is not physicallycross-linked, which means that chain mobility is not restrictedseverely. When the membrane is hydrated, water molecules enter thehydrophilic regions, increasing their size and shape, as well asmembrane ionic conductivity. The effect of these dynamic results is afairly narrow operating window for maximum fuel cell performance. At lowlevels of hydration, that is, low humidity and temperatures over 80° C.,the proton conductivity of Nafion® drops significantly. Other problemswith Nafion® membranes are high cost, high osmotic drag of water, andhigh methanol permeability.

[0003] One of Nafion's strengths as a fuel cell membrane is that itforms a microphase separated architecture upon film formation.Specifically, the film is composed of hydrophilic (water-loving) ionic“clusters” or “channels” dispersed in a hydrophobic matrix. At the sametime, because of the lack of covalent crosslinking, the optimalstructure for best performance can not be fixed, that is, the protonconductivity, channel size, and degree of hydration are dynamic andchange with operating conditions. Another problem with Nafion® membranesis that protons need water or other similar functional groups in orderto migrate through the membrane. There are no additional hydroxyl groupsin Nafion® which can carry out this function. The ether oxygen atoms inthe channels are flanked by strongly electron withdrawing CF₂ groups,which render the lone pairs of electrons on the ether oxygen much lesscapable of sharing with a traveling proton.

[0004] In general, current approaches to new membrane materials are toadd sulfonic acid groups to pre-formed aromatic polymers. There are someproblems with this approach. First, the acidity of these sulfonic acidgroups is usually much less than the fluorosulfonic acid groups inNafion®, making it more difficult to achieve comparable protonconductivities without resorting to extremely high degrees ofsulfonation, which can lead to mechanical and solubility problems withthe film. Second, there is no guarantee that sulfonation will result ina channel structure in a film. Highly aromatic, rigid polymers such aspolyimides, and polybenzamidazoles, etc., for steric reasons may not beable to adopt the necessary configurations for good proton mobilitythrough the film. Third, water is still necessary for proton transfer inthese films, as there tends to be no other functional groups presentwhich can hydrogen-bond with the proton and facilitate its transport.

[0005] It would be highly advantageous, therefore, to remedy theforegoing and other deficiencies inherent in the prior art. New blockcopolymers proposed herein would provide for an improved membranematerial for use in fuel cell devices, and in particular in polymerelectrolyte membrane fuel cells. Accordingly, it is an object of thepresent invention to provide a new and improved polymer electrolytemembrane characterized by a three-dimensional structure with poresaligned and functionalized for the efficient transport of protonswithout the need for significant additional hydration.

[0006] It is another purpose of the present invention to provide for anew and improved polymer electrolyte membrane in which methanolpermeability is reduced.

[0007] It is yet another purpose of the present invention to provide fora new and improved polymer electrolyte membrane with improved thermaland mechanical stability.

[0008] It is still another purpose of the present invention to providefor a new and improved polymer electrolyte membrane for use in fuel cellapplications that require higher methanol concentrations, therebyproviding for improved fuel utilization, enhanced cathode catalyticactivity, and reduced system complexity for water recovery.

SUMMARY OF THE INVENTION

[0009] Briefly, to achieve the desired objects of the instant inventionin accordance with a preferred embodiment thereof, provided is a polymerelectrolyte membrane and a method of fabrication comprised of ahydrophobic hydrocarbon region, a hydrophilic region containingcovalently bound acid functional groups and protic functional groups.The hydrophobic hydrocarbon region and the hydrophilic region arecovalently bound to form a single polymer molecule.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The foregoing and further and more specific objects andadvantages of the instant invention will become readily apparent tothose skilled in the art from the following detailed description of apreferred embodiment thereof taken in conjunction with the drawing, inwhich:

[0011]FIG. 1 illustrates the general structure of an asymmetric diblockcopolymer in accordance with the present invention;

[0012]FIG. 2 illustrates potential monomers for the preparation of blockcopolymers in accordance with the present invention; and

[0013]FIG. 3 illustrates a method of fabricating a novel polymerelectrolyte membrane in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] Recent developments in living polymerization processes,specifically quasiliving or controlled radical polymerization (CRP),have made it possible to improve fuel cell membranes in terms offormation of the fuel cell. In particular, membranes formed of blockcopolymers provide for a microphase separation upon film formation.Control of the resulting morphology will be achieved by optimizing themolecular structure of each block, the relative lengths of each block,and if necessary, through additional processing such as forming the filmon a patterned self-assembled monolayer coated surface. By synthesizingthe block copolymers using “controlled” radical polymerizationprocesses, the size of each block can be varied, and each will be ofnarrow polydispersity. In addition, these radical polymerizationprocesses are tolerant of a wide variety of functional groups.Consequently, molecular structure, density of hydrophilic functionalgroups, polymer Tg, and mechanical properties, can be definedspecifically. A more detailed description of the proposed chemistry ofthe polymer electrolyte membrane follows, beginning with a descriptionof the quasiliving radical polymerization process.

[0015] Block copolymers are increasingly important materials because ofthe properties which result from their unique molecular structure. Thesematerials combine the inherent properties of the parent homopolymersalong with the additional benefit of new properties appearing inrelation to the phase morphology. Block copolymers with narrowpolydispersity are known to undergo microphase separation as a result ofthe immiscibility of the two blocks. Bulk physical separation isprevented because the blocks are linked covalently. The resultingmorphologies are determined by the relative lengths of the two blocks,molecular structure of each block, molecular weight, and the magnitudeof the repulsive interactions between the chemically dissimilar blocks.Among the most observed morphologies are lamellae and cylindrical, whichcan be oriented either parallel to or perpendicular to the supportingsurface, and spherical. Several techniques, including self-assembly,have been reported recently for fabricating films or asymmetric blockcopolymers with a good deal of control over film morphology andorientation.

[0016] Referring now to FIG. 1, illustrated is a general structure of anasymmetric diblock copolymer 10 composed of a hydrophobic A block (solidline) 12 and a hydrophilic B block (dashed line) 14. It is proposed toprepare a series of asymmetric block copolymers, A-B, in which the Ablocks are hydrocarbon and may contain some level of a functional groupthat will undergo photochemical cross-linking in a subsequent step, anda hydrophilic B block containing fluorosulfonic acid groups and proticfunctional groups such as hydroxyl or amine. These new block copolymersundergo microphase separation upon film formation. More particularly,hydrophobic block 12 is formed as a hydrocarbon region. Hydrophilicblock 14 includes covalently bound acid functional groups and proticfunctional groups. The hydrophobic hydrocarbon region 12 and hydrophilicregion 14 are covalently bound to form a single polymer molecule, andmore particularly a block copolymer.

[0017] As previously stated, control of the resulting morphology will beachieved by optimizing the molecular structure of each block, therelative lengths of each block, and if necessary, through additionalprocessing such as forming the film on a patterned self-assembledmonolayer coated surface. By synthesizing the block copolymers using“controlled” radical polymerization processes, the size of each blockcan be varied, and each will be a narrow polydispersity. In addition,these radical polymerization processes are tolerant of a wide variety offunctional groups. After processing the film to achieve optimalmicrophase separation, the 3D structure may be fixed by photochemicalcross-linking within the A blocks.

[0018] The most versatile quasiliving radical polymerization system isthe atom transfer radical polymerization process (ATRP). Using ATRP, itis possible to polymerize a wide variety of monomers, even in thepresence of trace impurities, and generate a diverse set ofmacromolecular architectures. ATRP employs the reversible activation anddeactivation of the initiator, typically chosen from the group includingbenzyl chlorides and halogenated esters, such as ethyl 2 bromoisobutyrates, by transition metal catalysts to form radicals which canpropagate by addition of monomer. The molecular weight, or degree ofpolymerization, of the resulting polymer is defined by the ratio of theconcentration of reacted monomer to that of the introduced initiator:DP_(n)=Δ[M]/[I]_(O), and values between 200-200,000 are typical.Polydispersities are very low, usually between 1.04-1.5, and on averagecloser to the lower end of the range. Herein is described a method forpreparing a series of diblock copolymers for polymer electrolytemembrane fuel cell (PEMFC)/direct methanol fuel cell (DMFC) applicationsusing monomers 20 such as those illustrated in FIG. 2. Morespecifically, illustrated are a hydrophobic ‘A’ block 22, and ahydrophilic ‘B’ block 24. As illustrated, hydrophobic blocks 22 will beconstructed using monomers known to undergo radical polymerization, suchas any combination of the following monomers: styrene, 4-alkylstyrene,isoprene, acrylates, acrylamides, methacrylates, vinyl aromaticmonomers, and vinyl ether monomers. Cross-linking is optional and can beaccomplished in a subsequent step by photochemical irradiation. Theoptimal level of cross-linking will be determined throughexperimentation. Hydrophilic blocks 24 will be prepared using acombination of functionalized monomers such as those chosen from a groupincluding: hydroxylated acrylamides and acrylates, acrylonitrile,vinylamines such as 4-vinyl pyridine, monomers possessing one or moresulfonic acid groups such as 4-vinylbenzene sulfonic acid, and sulfonylfluoride substituted monomers capable of undergoing CRP. It isanticipated by this disclosure that other protic functional groups canbe incorporated in hydrophilic block 24.

[0019] Referring now to FIG. 3, illustrated is the chemistry proposed toprepare novel block copolymers for the polymer electrolyte membraneaccording to the present invention. In addition to the need to determinethe optimal level of crosslinking (ratio of n/m), it will be necessaryto investigate the optimal loading of sulfonic acid groups (ratio ofx/y) and finally, optimal size of B block relative to A block([(n/m)/(x/y)]; i.e., the density and morphology of nanochannels).

[0020] Using the chemistry proposed herein, it will be possible to notonly incorporate the now assumed best functional group for good protonmobility in the hydrophilic blocks, hydroxyl group, but additionallyother functional groups in the hydrophilic block and make a directcomparison of resulting proton conductivity.

[0021] Other functional groups that would be good candidates to comparewith the hydroxyl group include the: carboxylic amide, carboxylic acid,β-diketone, phenol, phosphoric acid, and amine. These alternativefunctional groups will be incorporated into the hydrophilic block duringradical polymerization by using the appropriately functionalized vinyl,styryl, acrylate, or acrylamidyl monomers in place of the poly-hydroxylmonomers.

[0022] Illustrated in FIG. 3, is the method 30 according to thisdisclosure for the processing of the polymer to form a membrane for fuelcell applications according to the present invention. As illustrated, afirst step in the method 30 includes providing for a initiator molecule32 and an unfunctionalized hydrophobic monomer 34. A reagent 36 isprovided, generally composed of a transition metal compound, solvent, orother similar material which provides for a reaction to proceed.Subsequent to the reaction 36, a functionalized monomer is formed 38,and then a block copolymer 40. As illustrated, the new block copolymer40 will be cast into films 42 using standard procedures such as solutioncasting, dip- or spin-coating. Mechanical, thermal, and structuralproperties of the as-deposited film will be determined, and if needed,film microstructure will be optimized by annealing at temperatures nearthe Tg of the film, altering polymer microstructure, changing the ratioof n/m, or by some other processing technique. Once the desired filmmorphology is attained the film can be photochemically crosslinked 44.Cross-linking should lock the 3D structure and provide increasedmechanical, chemical and thermal stability. In the final step, the filmwill be made proton-conductive by conversion 46 of the sulfonyl fluoridefunctional groups to sulfonic acid functional groups, using methodsdeveloped for the preparation of Nafion® films. Films of a variety ofthicknesses are anticipated by this disclosure.

[0023] Ideally, membranes for PEMFCs include low cost materials, havehigh mechanical, thermal, and chemical stability, and good conductivityover a broad temperature range (−40° C. to 150° C.) and in low humidityenvironments. With respect to DMFCs, the membrane should additionally beimpermeable to methanol, and demonstrate low levels of electro-osmoticdrag of water. Accordingly, as disclosed herein a polymer electrolytemembrane is disclosed providing for use of a higher methanolconcentration, reduction of complexity in an associated water recoverysystem, enhanced cathode catalytic activity and improved fuelutilization. The overall outcome is a fuel cell with significantlyimproved performance.

[0024] By now it should be appreciated that a novel polymer electrolytemembrane and method for making the membrane have been provided. Apolymer electrolyte membrane is disclosed which provides for improvedthermal, chemical and mechanical properties for applications in PEMFCsand other related applications such as electrochemical processes,electrochemical sensors, electro-chromic devices, batteries,supercapacitors, and the like.

[0025] The various steps of the method disclosed have been performed ina specific order for purposes of explanation, however, it should beunderstood that various steps of the disclosed method may beinterchanged and/or combined with other steps in specific applicationsand it is fully intended that all such changes in the disclosed methodscome within the scope of the claims.

[0026] While we have shown and described specific embodiments of thepresent invention, further modifications and improvement will occur tothose skilled in the art. We desire it to be understood, therefore, thatthis invention is not limited to the particular forms shown and weintend in the appended claims to cover all modifications that do notdepart from the spirit and scope of this invention.

What is claimed is:
 1. A polymer electrolyte membrane comprising: ahydrophobic hydrocarbon region; a hydrophilic region containingcovalently bound acid functional groups and protic functional groups;wherein the hydrophobic hydrocarbon region and the hydrophilic regionare covalently bound to form a single polymer molecule.
 2. The polymerelectrolyte membrane of claim 1 wherein the hydrophobic hydrocarbonregion is cross-linked.
 3. The polymer electrolyte membrane of claim 1wherein the hydrophilic region is cross-linked.
 4. The polymerelectrolyte membrane of claim 1 wherein the single polymer molecule is ablock copolymer.
 5. The polymer electrolyte membrane of claim 1 whereinthe single polymer molecule is an interpenetrating polymer network. 6.The polymer electrolyte membrane of claim 1 wherein the hydrophobichydrocarbon region includes at least one of a styrene, a 4-alkylstyrene,an isoprene, an acrylate, an acrylamide, a methacrylate, a vinylaromatic monomer, or a vinyl ether monomer.
 7. The polymer electrolytemembrane of claim 1 the hydrophilic region containing covalently boundacid functional groups and protic functional groups including at leastone of a hydroxylated acrylamide, an acrylate, an acrylonitrile, avinylamine such as 4-vinyl pyridine, a monomer possessing at least onesulfonic acid group such as 4-vinylbenzene sulfonic acid, or a sulfonylfluoride substituted monomer.
 8. A polymer electrolyte membranecomprising: a cross-linked hydrophobic hydrocarbon region; across-linked hydrophilic region containing covalently bound acidfunctional groups and protic functional groups; wherein the hydrophobichydrocarbon region and the hydrophilic region are covalently bound toform a block copolymer.
 9. The polymer electrolyte membrane of claim 8wherein the hydrophobic hydrocarbon region includes at least one of astyrene, a 4-alkylstyrene, an isoprene, an acrylate, an acrylamide, amethacrylate, a vinyl aromatic monomer, or a vinyl ether monomer. 10.The polymer electrolyte membrane of claim 8 the hydrophilic regioncontaining covalently bound acid functional groups and protic functionalgroups including at least one of a hydroxylated acrylamide, anacrylates, an acrylonitrile, a vinylamine such as 4-vinyl pyridine, amonomer possessing at least one sulfonic acid group such as4-vinylbenzene sulfonic acid, or a sulfonyl fluoride substitutedmonomer.
 11. A method of forming a polymer electrolyte membranecomprising the steps of: providing an initiator molecule; providing anunfunctionalized hydrophobic monomer; providing a reagent therebycausing the initiator molecule and the unfunctionalized hydrophobicmonomer to undergo a chemical reaction forming a sulfonyl fluoridefunctional group characterized as a block copolymer; casting the blockcopolymer into a membrane film; photochemically crosslinking themembrane film; converting the sulfonyl fluoride functional group to asulfonic acid functional group, thereby forming a polymer electrolytemembrane.
 12. A method of forming a polymer electrolyte membraneaccording to claim 11 wherein the step of providing an initiatormolecule includes providing at least one of a benzyl chloride intiatioror a halogenated ester, such as ethyl 2-bromoisobutyrate.
 13. A methodof forming a polymer electrolyte membrane according to claim 11 whereinthe step of providing an unfunctionalized hydrophobic monomer includesproviding at least one of a styrene, a 4-alkylstyrene, an isoprene, anacrylate, an acrylamide, a methacrylate, a vinyl aromatic monomer, and avinyl ether monomer.
 14. A method of forming a polymer electrolytemembrane according to claim 11 wherein the step of providing reagentincludes providing at least one of a transition metal compound or asolvent.
 15. A method of forming a polymer electrolyte membraneaccording to claim 11 wherein the step of casting the block copolymerinto a membrane film includes one of solution casting, dip casting, orspin coating.
 16. A method of forming a polymer electrolyte membraneaccording to claim 11 further including the step of optimizing themembrane film microstructure by annealing at a temperature near the Tgof the membrane film.